Existing powerful wind turbines have a turbine with three blades and a rotor. Each blade is attached to the hub via the large diameter roll bearing. This design does not allow to increase the number of blades in the rotor. Turbine having more than three blades in the bush does not run. The three-blade turbines have low efficiency of wind power, which is about 26%.
There are several reasons for the decline of efficiency. The main reason is that there is a preconceived assumption that Wind Turbine Power depends weakly on the number of blades and depends only on the swept area of the blades. When designing the turbine curve the Betz diagram is used and his postulate of the theoretical maximum ratio of wind energy which is equal to 0.59:2:6. FIG. 1 shows the theoretical curves of wind energy efficiency of an ideal wind turbine: ξ—wind energy efficiency: V1, V0—wind (air) flow rate through the wind turbine and at the turbine inlet. A. Betz in his dissertation considered changing the air flow kinetic energy in the profiled air channel with conditional walls. He did not take into account the mass-energy exchange with ambient air turbine air flow. The reduced efficiency of a rotor three-blade turbine still occurs due to the fact that the rotating air stream appears after the rotor which reduces the value of an air pressure drop in the rotor thus reducing the turbine output power.
A small number of blades leads to emergence of low-frequency air pressure pulsations. The pulsation frequency is in the zone of dangerous infrasound 0÷12, Hz. Pulsations in the single rotor turbine arise at moments of a blade passing past the tower.
Typically a human ear perceives pulsation within the range of 16-20000 Hz. Its unpleasant consequences does not only cause an excessive noise in the audible pulsation range but generate the unperceivable infrasound in the human hearing within the range from 16 Hz to 0.001 Hz. Infrasound causes nervous stress, malaise, dizziness, changes in the internal organs, especially in the nervous and cardiovascular systems. The most dangerous is the infrasound range from 6 to 9 Hz. Significant psychotropic effects are manifested most strongly at the frequency of 7 Hz which is consonant with the alpha rhythm of the natural oscillations of the brain. Any mental work in this case is impossible. The sound of low intensity causes nausea and ringing in the ears, blurred vision and irrational fear. Medium intensity infrasound causes the frustration of digestive organs and the brain giving birth to a paralysis, general weakness, and sometimes a blindness.
Pressure pulsation and therefore the speed deviation result in some loss of turbine power due to the reducing cyclic pressure differential across the turbine wheel. The loss of power could be as high as 2-5% of its theoretical value. The experimental results published by the Massachusetts University show that the significant part of the sound energy is in the audio frequency range from 0 to 20 Hz, as shown in Example 3. Therefore, from both the economic and environment protection viewpoints it is advisable to eliminate the infrasound.
Conventional technical solutions for the elimination of the cause of the low-frequency oscillations at the wind turbines (wind power installations) are increasing the number of blades or an increasing the rotation speed of the wind wheel.
The double-rotor turbine rotors generally rotate in opposite directions. Their pressure pulsation frequency is higher than that of a single rotor turbine. The infrasound does not depend on the influence of the tower does. However, the pressure pulsations occur from the influence of the blades of one rotor on the blades of the other one. The freely selectable number of blades in the rotors can lead to an effect of acoustic beat. Therefore, the number of rotor blades Should not obey to the same multiplication factors. For example: one row of blades has four blades, the second row has two blades. Their total multiplier is 2. Similarly, one rotor has six blades and the other one has three. Their total multiplier is 3. In both cases the infrasound in the 4-6 Hz frequency range shall be generated.
The acoustic beat phenomenon can be eliminated if the number of blades in one rotor is equal to Z, and in the other is (Z+1).
The wind turbine is known according to the patent application [JP 2005036749 A 10 Feb. 2005], where:    Claim 1. The horizontal axis turbine rotor with variable speed and rotatable in azimuth, depending on the wind speed. The wind turbine provided with the means for measuring and recording the azimuth angle, rotation speed and the noise measurement system for recording time and frequency.    Claim 2. Wind turbine according to claim 1, with rotor speed controls.    Claim 3. Wind turbine according to claim 1, where the speed control is performed at the particular moment of time.    Claim 4. Wind turbine according to claim 1, where the controller device controls the azimuth angle of the rotor speed depending on the low-frequency noise.    Claim 5. Wind turbine according to claim 1, where the speed control is performed in the specific area at the turbine leeward or windward side.    Claim 6. Wind turbine according to claim 1, where the rotation speed is reduced at the particular time in the particular area for suppressing the low-frequency noise.
In the patent application JP 2005036749 A, both the claims and in the description refer to the low-frequency noise. This invention does not comprise any relation to the method for the prevention or elimination of infrasound. The method is offered to reduce the action of low-frequency noise on a specific area by the rotation of the noise exposure vector or by reducing the wind turbine rotation speed or its operating hours.
There are the two different notations of «infrasound» and the low-frequency noise.
Presently, the generation of infrasound by all wind turbines is a serious environmental problem.
Patent RU 2463475 C2 10 Oct. 2012 represents the wind power plant comprising the wind turbine placed on a tower, the two coaxial multi-blade propellers with a horizontal axis and a rotary body, an electric generator and a step-up gear. The step-up gear is connected to the shaft of the electric generator and the propellers shaft. The propellers control system controls the installation angles of the blades. The propellers are mounted at the same side of the rotational axis on their coaxial shafts and they are provided with the number of blades selected on the condition z1·z2≤f/ωc, where z1 and z2 are the number of blades if the first and the second propeller, respectively; f is a safe infrasound frequency which is at least 10 Hz; ωc=ω1+ω2 is the relative rotation speed of the wind turbine, and ω1 and ω2—the frequency of rotation of the first and second propeller, rps.
The disadvantages of this system are as follows. The formula for the optimization of the number of blades which is aimed to avoid the emergence of infrasound does not account for the possibility of acoustic beat in the infrasound frequency range. Acoustic beat can occur due to the coincidence of the phase position of two or more blades.
The conventional wind power plants from which the generated electricity is transmitted to an electrical network implement usually the alternate current three-phase current generators. Their diversity is reduced to two types, namely, the synchronous generators and asynchronous generators. In both cases, as a rule, the mechanical step-up gear should be installed between the wind turbine and the electric generator. Qualitative characteristics of generated electric current and optimum use of wind power requires a constancy of the power generator rotor speed. Constant speed is easily established if the wind speed is either nominal or above nominal. This occurs due to the automatic positioning of the turbine blades. If the wind speed is reduced to 10 m/s and below, maintaining a constant the number of revolutions of the generator shaft becomes a problem. The wind turbines usually have an additional complex electronic equipment in the form of a frequency converter, its output is very consistent with the frequency harmonic of the electric power network.
The double-rotor installations the planetary gears are mostly used as step-up gears, which are installed in the wind, turbine transmission between the propellers and the electric generators. Planetary transmission having three movable shaft are commonly referred to as the differentials.
All existing differentials obey one and the same general rule. At a constant rotation speed of one certain shaft the rotating speeds of all other shafts depend on the torque applied to each such shaft. If the number of revolutions of the shaft increases, the rotation speed of other shaft is reduced, and vice versa. Let us consider the use of such a differential in the counter-rotating wind turbine. The reducing the wind speed causes the lower wind energy imposed on the turbine blades. The speed of turbine rotors begins to decline. The blade angle automatic control installation slows down the speed reduction, but it cannot restore this speed to its to initial value.
These processes are shown for convenience in FIGS. 2 and 3.
FIG. 2 shows decrease of the turbine speed with a decrease of the wind speed.
FIG. 3 shows variation of the speed of a special three-shaft planetary mechanism (the differential).
With the decrease of wind speed causes the turbine speed decreases, the speed of the second rotor 2 decreased less intensively than that of the first rotor 1 (which is explained by FIG. 2). When the speed of any rotor, for example, the first rotor 1 falls, than in order to maintain the generator speed at the nominal constant level it is necessary to increase the speed of the second shaft 2 (which is explained by FIG. 3). But this is impossible, since the energy of the air flow with the decreasing wind speed is also reduced.
If the rotor speed of the generator decreases according to the decreasing wind speed, the generator automatically switches to an electric motor mode powered from the central power supply network. Generation of electricity is stopped. This usually occurs at wind speeds lower than 20-25% of the nominal value.
The device is necessary that would support the momentum of the generator at the lowest possible wind speeds, which would provide a high power output.
The wind turbine is known under the patent application FR 2589201A1 30 Apr. 1987. The wind turbine with two rows of counter-rotating rotors, where the claim 1 comprises:                1) a supporting tower;        2) two freely rotating rotors with blades and their position control mechanism;        3) the propeller control unit which utilizes the wind energy for the rotation of the blades.        
Claim 2 of the same patent comprising the wind turbine according to claim 1, where the propeller control unit utilizes a pair of ball screw pair.
Claim 3 of the same patent comprising the wind turbine according to claims 1 and 2, with the mechanisms which convert the longitudinal movement of the propeller control unit into circular motion of the blade pins.
Claim 4 of the same patent according claims 1, 2, 3 where the upper (front) rotor rotates freely relative to the lower (rear) rotor, while the lower rotor is freely rotatable relative to the output shaft.
However, this structure does not correspond to the above statement. Upper and lower rotors are linked with gear-coupling with fixed axes. Therefore, the rotors cannot rotate freely at all. The torque from the upper rotor is applied with the gear transmission to the lower rotor, and only after that it is applied with the gear transmission to the output shaft.
Such kinematics has only one degree of freedom and therefore, the functional twin-rotor system turns into a single-rotor one with the dependent constrained rotors rotating in opposite directions. A stop of any element stops the entire system. Thus it is physically impossible to maintain a constant rotor speed using the positioning of the blades when the wind speed changes.
In the patent application FR2589201A1, in claim 5, the gear drive according to claim 4 is disclosed between the upper and the lower rotors and the output shaft.
The description contains certain phrases, e.g., « . . . it contributes to maintaining the rotation speed of the rotors irrespective of changes of the wind speed»; «nearly constant rotor speed»; «This is facilitated by the rotation of the blades around their axes», etc. Such statements are the result of the general discussion. The proposed design scheme of the transmission from turbine rotor to the shaft can not provide the constant output shaft speed.
Wind turbine under the patent US 2006/0093482 A1 4 May 2006 is known. This twin-rotor turbine has coaxial rotors. The set includes a planetary type step-up gear and a special braking mechanism for the entire system.
The turbine utilizes a simple planetary satellite gear. Reducing the wind speed below the nominal, to approximately 20%, the rotation speed of all rotors is also reduced. Accordingly, the rotation speed of the generator is also reduced with all adverse effects.
The patent application JP 2007321659 A 13 Dec. 2007 describes the double-rotor installation in which the coaxial 3-blade rotors rotate in the same direction. Actually there is only one rotor in which the turbine wheel is turned relative to the other wheel to a certain angle depending on the wind speed. It is assumed that this method can achieve higher efficiency of wind energy. The mechanism is disclosed for turning one rotor to a certain angle relative to the other wheel.
However, the double-rotor turbine comprises a serious structural problem, namely, the inner shaft is thinner than the outer shaft. To prevent the physical contact of the rotors when the nacelle turns around its axis and the tower, the inner shaft should be extended. It results in the increased distance between the rotors. The deflection of the shaft increases, its strength decreases.
The layout of the existing wind turbine installations is mostly the same. The power generator is installed in the nacelle. The nacelle rotates on the axis of the tower depending on the wind direction. From the generator the cable is passed down along the fixed tower. When the nacelle rotates the cable is twisted. It is necessary either to do any automation for cable counter-twisting with the rotation of the nacelle or install special current collectors in the cable cut. In any case, it is a problem that reduces the reliability of the system, which increases the cost of installation and its maintenance.
A brief review of the existing wind turbines shows the following disadvantages:                a) low efficiency of the use of the wind (airflow) energy in comparison with gas or hydro turbines;        b) generation of infrasound, of a low frequency sound in the range 0-12 Hz;        c) failure to maintain the constancy of the electric generator rotor speed at lower wind speeds below 20% of its nominal speed. This is usually below 8 m/s:        d) twisting of the main power cable by the nacelle rotating on the stationary tower.        
The technical result is the better use of wind enemy, the increase in electricity generation, shorter maintenance intervals, versatility of manufacture and operation, increased reliability and service life.